This invention relates to magneto ignition systems for small internal combustion engines of the type having a magnet in the flywheel or rotor that is driven in synchronism with the engine operation to generate the ignition spark through a primary and secondary winding coil arrangement in which the magnet generates primary current.
In a typical magneto ignition system of this type, the rotation of the rotor causes the magnet to induce a current in a primary winding that is located adjacent to the rotor or flywheel. This current flow is controlled by a switch device which allows the current to flow as the field from the rotor increases and which stops the current flow abruptly at a particular point in the engine cycle at which time a self-induced primary winding voltage is established. This produces a larger voltage, the ignition spark, upon a secondary winding that is electromagnetically coupled to the primary winding. In general, the spark voltage is related to the primary winding voltage in proportion to the turns ratio between the primary and secondary windings.
One form of a switch commonly used to control the primary winding current is simply a breakerpoint that is operated through the engine crankshaft to open and close at the proper times in the engine cycle to turn the current ON and OFF. At the time of opening, however, the self-induced primary winding voltage, which can be on the order of 400 volts or more, appears across the points, and despite the use of capacitors, there is gradual erosion or pitting of the contact surfaces. In addition, the rubbing block or interconnecting hardware between the breakerpoints and the engine output shaft also undergoes gradual wear. The wear and the point erosion can cause a gradual change in engine ignition timing and this gradual change in timing can result in loss of power and deteriorated fuel economy.
Consequently, semiconductor switch devices are also used to substitute for the breakerpoint arrangement since they are not subject to those mechanical problems. The function remains the same, however: to turn the primary winding current ON and OFF at the proper times during the engine cycle so as to produce the spark at or near the top of the compression stroke. It is also desired that the necessary advance characteristics for efficient engine combustion be provided. Proper switching requires that the primary current be allowed to build as the magnet field increases in the primary winding and that it is turned OFF near its maximum level so as to generate the largest possible ignition spark.
A transistor switch, however, is resistive and therefore tends to lower the maximum current flow in the primary circuit. Its resistance can be minimized, of course, by driving the transistor into saturation. But there is the problem, however, with those devices where the transistor switch is controlled by a bleed resistor between the collector and base input. The bleed resistor produces a voltage drop between the collector and base to insure operation of the transistor in its active region. But the result of this voltage drop is that the saturation voltage of the transistor is raised, which thereby reduces the primary winding current.
Some approaches attempt to avoid this problem by using a separate coil to generate the base drive for the transistor switch. In particular, the base drive coil is connected between the base and emitter of the transistor switch and produces a voltage in response to the flywheel rotation to place the switch in the active state to allow a current flow, that occurs in a phase relationship with the primary winding current since the base drive coil is wound on the same magnetic frame with the primary winding. This is also necessary to insure that the primary winding current reaches its maximum level. A problem associated with this arrangement, however, is that the coil must be constructed so as to have the capability to supply the necessary current from the magnet to excite the transistor into a saturated conducted state, which as mentioned previously, is necessary to achieve the maximum possible current flow. A power transistor is normally used and these are characterized by an extremely low base to emitter current gain characteristic. The need for a power transistor arises simply from the fact that the primary winding current can be comparatively high, for example, several amperes. Because of a power transistor's extremely low current gain characteristic, however, the base drive coil must be constructed to produce a significant amount of current. Thus the windings can be significantly large, which increases the overall size of the ignition system.
In other applications, a gate-turn off device is used instead of the simple power transistor. The advantage of the gate turn off device is its low active voltage drop which therefore maximizes the primary winding current. With this device, the bias winding is connected to the gate and the bias winding voltage generating the gate current to turn the switch ON, which allows the current to flow through the primary winding. When the bias winding output voltage drops below a particular negative level, the gate current is suddenly reversed which turns the switch OFF and thereby abruptly stops the current flow which produces the ignition spark in the same way discussed above.
The gate controlled switch, however, possesses some undesirable characteristics. Among these characteristics is that the current gain relationship between the gate input and the anode-cathode output, is such that more current is needed to turn the switch OFF than is needed to turn it ON. Consequently, the current gain, when the switch is conducting is much smaller than the current gain when the switch is not conducting. Because of this, the bias coil must be designed to have sufficient capability to supply the necessary current to turn the device off. The bias coil, therefore, is rendered somewhat larger than it might otherwise have to be if the current gain characteristics were uniform. The ignition system is thus larger also.